1. Field of the Invention
This invention relates to pulsed or continuous lasers employing second harmonic generation, sum frequency generation, or difference frequency generation. More specifically, the invention is a method for controlling/optimizing the efficiency of a laser that employs second harmonic generation.
2. Description of the Related Art
Pulsed and/or continuous lasers at a variety of specific wavelengths are known and commercially available. Nonlinear interactions between available laser beams are commonly used to generate laser beams at wavelengths for which no laser is available. Briefly, nonlinear optics are used in conjunction with a commercially-available laser(s) to generate laser frequencies at the sum and difference frequencies of the laser beams input to the nonlinear optics. A special case of sum frequency generation is known as Type II second harmonic generation in which the nonlinear optics receive an input beam with two polarizations. In other words, the input beam is essentially two input beams that have the same frequency and direction of propagation, but different polarizations. The nonlinear optics cause the two input beams to interact in a volumetric fashion to generate an output laser source at the sum frequency that is twice the laser frequency.
A performance metric of such second harmonic generation laser systems is the conversion efficiency of the nonlinear interactions that is often limited by the spatial and/or temporal separation of the two beams that are input to nonlinear optics. For efficient conversion, the laser beams associated with each polarization and interacting in the nonlinear optics must overlap both spatially and temporally. Pulsed laser beams that are completely overlapping when they enter the nonlinear optics can separate temporally (in the nonlinear optics medium/element) because the group velocities of the two beams are different. Pulsed or continuous laser beams that were initially completely overlapping could also (or alternatively) separate spatially because the direction of the group velocities of the two beams are different. Further, as the optical length of the nonlinear optical medium/element (e.g., a nonlinear crystal) increases, so does the temporal and/or spatial separation. Accordingly, these attributes limit the useful length of the nonlinear optical medium/element that can contribute to the conversion efficiency. Because efficiency often depends on the useful length of the nonlinear optics squared, attributes that limit the overlap of the interacting beams also severely limit the conversion efficiency.
Currently, nonlinear devices employing Type II second harmonic generation rely on the input beams of laser pulses being completely overlapped when input to (or incident on) the nonlinear optical medium/element. This happens naturally in cases where both input beams of laser pulses are generated by the same source, e.g., as is the case in Type II second harmonic generation. However, the two beams begin to separate as they travel through the nonlinear optics. Nonlinear optics that limit interactions to a short optical path degrade the conversion efficiency. For nonlinear optics with a longer optical path length, the two laser beams can completely separate thereby causing all conversion to cease. While lateral spatial separation can be mitigated to some degree by employing a pair of opposing nonlinear crystals (e.g., each half as long as the desired length), this approach introduces phase control problems of the second harmonic as the laser beams travel between the pair of nonlinear crystals.